As known, in the paper converting industry a variety of machines and processes are used for making paper napkins, wipers, certain types of toilet paper and other articles in the form of folded and/or interfolded stacks of sheets.
A production process starts from a web having a certain transversal width, from which sheets are obtained, that are folded, stacked continuously and then split into stacks of a same height as the final packages. Each of the stacks is called a log and has the same length as the above described transversal width. The logs, moreover, are cut off and wrapped into many packages with determined final size.
In some cases, the sheets obtained from the web are folded separately from one another and then stacked already folded. In other cases, the sheets are interfolded, namely are folded into panels by overlapping at the same time a panel of a previous sheet with a panel of a following sheet. In this way, when pulling a sheet from the stack, at the moment of the use also a panel of the following sheet is pulled out, with consequent advantages for certain types of uses. Among the possible interfolding ways, stacks of L, Z or W interfolded sheets are known having 2, 3 and 4 panels respectively.
For the production of interfolded stacks, machines are known that use one or two webs of paper coming from a reel that are cut into sheets and then supplied offset with respect to one another on folding counter-rotating rollers.
More precisely, the webs are cut into sheets by means of cutting rollers that engage with respective blades. In case of L or W interfolding, the webs are cut so that they form a sequence of offset sheets coming preferably from two different directions. Therefore, the sheets coming from both directions are supplied alternately to the folding rollers so that each sheet coming from a first direction overlaps a portion of the sheet coming from the second direction, and vice versa.
The sheets coming from both directions, in order to be overlapped in the above described way, adhere to the respective folding rollers by means of a vacuum-suction step or by means of mechanical gripping.
In machines where mechanical clamps hold the paper, special folding rollers 101 are used, as shown diagrammatically in FIGS. 1 and 2. In particular, a folding roller 101 of prior art is circumferentially equipped with a plurality of folding elements 103 alternated to a plurality of mechanical clamps 104. In particular, each mechanical clamp 104 consists of a fixed jaw 104a and a movable jaw 104b operated by a cam mechanism.
The folding process of the sheets or of the web of paper provides two counter rotating folding rollers that contact along a line where a folding element 103 of one roller forces a sheet or web of paper into a mechanical clamp 104 of the other roller according to a predetermined combination of movements. The alternation of the folding elements 103 and of the mechanical clamps 104 on the boundary of the folding roller 101 causes the sheet or web of processed paper to fold according to different possible configurations.
More in detail, when mechanical clamp 104 is at a first predetermined angular position, movable jaw 104b is operated by a cam and pushes against fixed jaw 104a the sheet, which engages on the surface of folding roller 101. Then, when then mechanical clamp 104 is at a second predetermined angular position, the cam mechanism opens mechanical clamp 104 releasing the sheet that then leaves roller surface 101.
Therefore, the sheet or web of paper is held on the folding roller in a certain angular portion defined between the above-described angular positions. Then, the downstream end of each sheet leaves its own folding roller at the contact line between the two rollers, adhering to the other folding roller, to which also the upstream end of the previous sheet adheres.
In addition to mechanical sheet holding systems, vacuum holding systems are known where the sheet of paper is caused to adhere to the folding roller surface by a vacuum-suction system. Furthermore, mixed vacuum-mechanical systems are known.
Both in the case of folding rollers with mechanical clamps and of vacuum folding rollers the need is felt for increasing the interfolding sheets' width, obtaining longer logs to cut-off for increasing productivity of the process and for reducing production costs. This would require an increased length of the rollers, which is impossible technically.
In fact, one technical reason is that the diameter of the rollers should also be increased proportionally, but this is impossible because the diameter is strictly given by a determined linear development necessary for interfolding. On the other hand, even maintaining the same diameter, the length of the rollers cannot be increased, since they are already weakened for the presence of circumferential grooves, and during the production process they are already subject to an inevitable bending, which has to be minimum for not damaging the machine and/or the product. Therefore, folding rollers with increased length cannot be made beyond a certain limit, because this causes the roller to bend too much, thus limiting the maximum length of the log at the outlet of the machine, then limiting productivity.